The Fallacy of Time Travel

The desire to know what will be, to rattle the cage of the future, of time itself, is close to being a human obsession. Of all the dimensions, it is time – a perpetually dangling bait – that we cannot do anything about. Stories of time travel are a manifestation of this frustration and the concomitant belief that it is possible to get to the future “if only.” In science fiction, the ‘if only’ is usually materialized through some sort of mechanical device that allows people to make that time journey. Einstein’s special law of relativity adds theoretical hope to that possibility, since it shows that time runs faster or slower depending on one’s location in space. It is the reason physicist Brian Greene states categorically that “time travel is absolutely possible” (1) and that “relativity lays out a blueprint for time-travel to the future.” (2) Unfortunately, it doesn’t. We can’t slow down or speed up the rate of our personal time. Our clocks and watches do not go slower or faster for us, no matter where we are or how fast we travel. Clocks moves faster or slower, not for each of us, but relative to an observer. Putting it more verbosely, a clock moving relative to an observer ticks more slowly than an identical clock at rest relative to that observer.

Any notion of time in the universe needs to be seen in the light of that principle. So if you could ever speed away to the stars at close to the speed of light and come back after five years, although everyone on earth would be fifty years older, you would still have aged only five years. And, yes, it might be nice to note that you had added on only five years while everyone else was fifty years older, but the fact is that you would not have really changed anything in your personal time span; you would still be only five years older than you were when you left. And sadly, despite the fact that everyone you knew on earth would be 50 years older than when you had left 5 years previously, it would not mean that you would live longer than had you stayed on Earth; you would live out your lifespan, with no connection to what had happened on Earth in your absence.

In fact, you wouldn’t even think of it as having “traveled in time” if there weren’t anything to compare it to; if you weren’t later in contact with those who had aged at a different rate. That time is relative is almost a cliché but not, it seems, when spinning the tale of time travel. In a colorful video, particle physicist Brian Cox says that “moving clocks move slower.” (3)They don’t. They do so only in comparison with other clocks moving at different speeds. What Cox’s demonstration shows is that “time is personal,” as he points out, but not that the person who is the subject of Cox’s demonstration “has traveled in time.”

As we have seen, our getting five years older in outer space while everyone else’s age increases by fifty years here on Earth, has to do with our own personal time. It is not time travel. Imagine two friends, Mary and Carol, both 25 years old. Mary was the lucky astronaut who traveled into space for five years at close to the speed of light. When Mary returned to earth she was 30 years old and found Carol to be an old woman of 75. That would be quite a shocking reunion, but somewhat mitigated by Carol thinking she could ask Mary – who, theoretically, had time-traveled – what was going to happen in the future, perhaps to know the winning lottery numbers that were still to come. Mary would look at her in amazement, for lottery numbers that had come up were obviously not in Carol’s future. But in a strange twist, it seemed to Mary that it was Carol, who had traveled in time; after all, Carol had lived through 50 years while Mary had been through a mere 5 years. And they would both be amazed to hear Brian Greene stating that “If you wanted to leapfrog into the future, to see what the world will be like a million years from now, it could theoretically be done.”(4) Mary would, in fact, have no idea what Carol’s future will be, since she had not traveled to the future. Time had not gone more slowly for Mary. When she came back after five years, those five years were in her past, her memory.

The notion that there is such a journey into the future is patently ridiculous. Mary didn’t travel forward in Carol’s time, or indeed in anyone’s time. If in 2020 she traveled close to the speed of light for 5 years, for her it would be 2025, while for Carol it would be the year 2070. So the notion that one could travel into the future and tell those left behind what the future would be, is preposterous. In fact, when Mary got back, Carol would already have passed that year 50 years earlier! Mary didn’t travel forward in her time any differently to what we all do as time passes. Neither of their clocks moved faster or slower.

To further illustrate how nonsensical the time travel notion is, had Mary moved forward in time, we could likewise claim that she subsequently moved back in time when she returned to Earth, so changing the future that she had just been to! In fact, where she had been was in the past for her, not in the future. And it certainly wasn’t Carol’s future. By the same token, we could say (if we wanted to further mangle our minds) that for Carol to then do the same trip would be for her to travel to Mary’s past.

If the mind game (which is what time travel is) just set out was confusing, it shouldn’t be surprising. The fact that it is deemed to be theoretically possible, however, should be. Traveling back in time is generally accepted as impossible, due to certain implications, like the “grandfather paradox.” Imagine going back in time and murdering your grandfather, before you were born. If you’ve killed your grandfather, you’ve prevented your own existence, and if you never existed, how could you have traveled back in time in order to do it? If that means that backward time travel is impossible, why would physicists conceive forward time travel as theoretically possible? The implications would be just as absurd, since the paradox of backward time travel is inherent in Mary’s venture into space: If Mary died after returning to Earth, supposedly “from the future,” she wouldn’t have been able to make the journey, not having been been alive in order to do it, so presenting us with another version of the paradox.

Beneath the concept of time travel is the implication that the future is where we have not yet arrived, This is, in fact, what Einstein’s theory of special relativity suggests: by destroying any notion of universal time and having time relative to location in space, the future is supposedly already out there waiting for us, and which we simply can’t see until we get there. “Are we there yet, are we there yet, are we there yet?” asks Bart over and over in a car ride with his family in the television cartoon series The Simpsons. If the fact that we are moving linearly towards the future implies that we will eventually arrive, it has no connection to time travel as is popularly envisaged. If it is time travel, though, it means that it is – and always has been – in Mary’s future to make the journey: predestined, in other words.

Underlying it all is the covert presumption that there is a future real or correct time that can be used as a marker to show that time moves faster or slower. But there is no universal time; time depends upon where one is. Time is always in relation to someone else’s time, so Mary’s time is always relative to Carol’s time. If, when Mary was traveling away from the Earth, the Earth was destroyed, she could not be considered as traveling in time, since there would be no way of comparing. In any case, there could never be a way of comparing two times; she would simply be 5 years older. There would, in other words, be no observer to make the comparison.

And that brings us to the matter of the observer, There has to be one, since the clocks move faster or slower relative to an observer. But – and this is a big but – it would not be possible to communicate with someone in another “time zone,” because the length of time it took for the communication to travel between two time zones would equal the gap in time between them. There could never be an observer “in real time,” because (1) there is no one real time, and (2) there is no location that an observer could be in order to see both locations simultaneously (in our case, Carol and Mary), since those times could not be simultaneous.

It’s time, then, to put to rest myths regarding time travel. As much as we would like it to happen, it will not be as physicist Paul Davies (who has written extensively about time travel) says, namely that perhaps in 100 years it will be normal to travel in time (5). Thus, when he asks “Might time travel one day become commonplace too?” the answer should be a resounding “no.”

Like Santa Claus, time travel to the future is an idea, pleasant in its promises. But unlike Santa Claus, which we eventually grow out of, the romantic possibilities inherent in traveling to the future, from The Time Machine of H. G. Wells to TV’s Doctor Who, have never been put out to pasture. The question is not whether it will ever be technically possible, but whether the notion “travel to the future” itself makes sense, even in terms of science fiction.

The problem with time travel is, as pointed out, inherent within a fundamental misstating of how time works according to Einstein’s general law of relativity. It is where a populist and romantic view of time travel prevails over the more pedantic actual physics.

It has been stated as fact that time goes faster at a higher elevation than at a lower elevation on Earth. Physicist Carlo Rovelli tells us in his books and talks for general audiences that “it is a fact that a watch positioned higher will go faster than a watch lower down.” This is measurable, he says, and goes on to say that it can be measured by a super-sensitive atomic clock, even when the difference in height is only 30 centimeters. This fact, he says, has nothing to do with Einstein’s theories. (6) But it is not fact, and it has everything to do with Einstein’s theories.

While time does go faster on Earth at a higher elevation, relative to an observer, it does so due to gravitational time dilation, a consequence of Einstein’s theory of general relativity, which posits that the gravity of a massive body – such as the Earth – warps the space-time around it, causing the flow of time to speed up or slow down depending on its distance from the mass.

For Jack at the top of a mountain, 5 minutes will be 5 minutes on his watch, as it would be for Bill in a valley. Jack’s clock will have moved faster relative to Bill’s, and this will be seen by comparing atomic clocks brought together from each, albeit too small to be ordinarily noticeable. For differences in time to be noticeable, the process could be sped up by Jack or Bill flying into space at close to the speed of light – and we are back to our “time travel” saga with all its attendant fallacies.

It all comes down to the fact that that no two events happen at the same time, and clocks tick at different rates according to where they are in relation to an observer. We don’t, then, need to get into the realm of time travel or the notion that higher clocks tick faster than lower ones to understand the principle that all clocks run at different rates, depending on where the observer is located. In other words, clocks are accurate only to themselves! When clocks are coordinated to give an accurate time – as they are for GPS – the clocks on a number of satellites in orbit above Earth are averaged to an incredible accuracy in order to provide location information anywhere on or near the Earth’s surface at a point on the Earth, i.e. the observer.

We can think of the observer as an independent arbitrator. And the fact that each observer is an independent arbitrator provides another perspective of what is meant by “time is relative”: it is different for each observer, who is necessarily in a different location from every other observer.

It is clear why people do not time travel when they speed away at close to the speed of light. And it is clear why people who are at a higher location do not grow older faster than those living lower down. The simple point is that their personal clocks run at the same rate for each of them; they do not speed up or slow down.

A clock, then, is never wrong. It is wrong only in the sense that it (always) shows a different time than on another clock. This should be clear from Einstein’s special theory of relativity, in which there is no universal time, and hence that time is uniquely personal; the relativity of time, being observer-bound, does per se not encompass notions of right or wrong time.

The uniquely personal implications for time travel are part and parcel of the theories of relativity. Even if there is as yet no consensus as to what time is and/or how it can be defined, or even if time does not necessarily exist within physical equations, surely the very notion of time travel can be brought up only by discarding the relativity of time. Failing that, it seems that we will continue to frustratingly dream on about traveling to the future and thereby extending our lives.

Ronald Green is a former lecturer in linguistics and philosophy at Tel Aviv and Oxford, the author of Time To Tell: a look at how we tick (iff Books 2018) and Nothing Matters: a book about nothing (iff Books 2011), and 13 ESL books used worldwide. He has lectured and given workshops in Europe, North and South America and the Middle East on linguistics, philosophy and the use of the Internet in education. His articles on philosophy have appeared in a number of journals, while his short stories have been published in several literary journals. He is active in taking philosophy down from “the ivory tower,” showing its connection to science, and explaining it in terms that are popularly understood.

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It may well be true that “a populist and romantic view of time travel prevails over the more pedantic actual physics.” And it may well be that respected physicists have made statements to general audiences which are misleading in certain respects. I am puzzled by some of the claims made here however.

“[P]hysicist Brian Greene states categorically that “time travel is absolutely possible” and that “relativity lays out a blueprint for time-travel to the future.” Unfortunately, it doesn’t.”

You insist the proposed example is not time travel. But it involves a traveller finding herself in a future (Earth-time) which would normally (i.e. without near speed of light travel) have been inaccesible to her. Calling this “time travel” seems not unreasonable.*

By way of explanation of your point of view, you continue: “We can’t slow down or speed up the rate of our personal time.” But who said we could? It seems to me that you are changing the subject here.

“That would be quite a shocking reunion, but somewhat mitigated by Carol thinking she could ask Mary – who, theoretically, had time-traveled – what was going to happen in the future, perhaps to know the winning lottery numbers that were still to come.”

I don’t see why Carol would think this.

“And they would both be amazed to hear Brian Greene stating that “If you wanted to leapfrog into the future, to see what the world will be like a million years from now, it could theoretically be done.” Mary would, in fact, have no idea what Carol’s future will be, since she had not traveled to the future.”

But she did travel to the future where she found her friend, old but still alive. The point is, she can’t now go back to 2020 and tell Carol.

“Carol’s Time had not gone more slowly for Mary. When she came back after five years, those five years were in her past, her memory.”

I don’t get this.

“The notion that there is such a journey into the future is patently ridiculous. Mary didn’t travel forward in Carol’s time, or indeed in anyone’s time. If in 2020 she traveled close to the speed of light for 5 years, for her it would be 2025, while for Carol it would be the year 2070.”

But does she not return to earth in 2070 (Earth-time)? I don’t see what your claim that “for her it would be 2025” means. The calendar is (like language) not a private thing.

We can accept that what Mary did is a real possibility without believing that any life-extension is involved. In fact, this seems to me the most natural way to understand it.

“So the notion that one could travel into the future and tell those left behind what the future would be, is preposterous.”

Yes, but who is claiming this? Greene is saying we can travel to the future. He is not saying we can then return (i.e. go “back in time”) and tell what we found.

“Time is always in relation to someone else’s time, so Mary’s time is always relative to Carol’s time. If, when Mary was traveling away from the Earth, the Earth was destroyed, she could not be considered as traveling in time, since there would be no way of comparing. In any case, there could never be a way of comparing two times; she would simply be 5 years older. There would, in other words, be no observer to make the comparison.”

If the earth had been destroyed, say, 20 years after her departure she would still have learned something about the future of the planet which she would not have known had she stayed at home.

* Ordinary usage is often out of line with scientific ways of understanding phenomena. We don’t bother putting scare quotes around sunrise or sunset, for example, or worry too much about grammatical constructions which seem to impute agency to non-agentive phenomena or inanimate things (wind blowing, windows breaking, doors closing, etc.). As I said above, calling what Mary does “time travel” seems okay to me, though I do see your point that the phrase could mislead. You are not traveling through the medium of time (on the model of air travel or sea travel).

Mark,
“But she did travel to the future where she found her friend, old but still alive.” (…) “But does she not return to earth in 2070 (Earth-time)? I don’t see what your claim that “for her it would be 2025” means. The calendar is (like language) not a private thing.” No, for Mary it is 2025; she returns to a culture with a different calendar than she carried with her, and she will no doubt agree that it is ‘now’ (for the culture she participates in) 2070. But that is not her future, that is simply another day forward through what her spaceship calendar would read as 2025. Carol, could be said to have aged 50 years in Mary’s five years; from Mary’s perspective the Earth has simply aged more rapidly.

If the world were destroyed in (Carol’s) 2040, then Mary would still not know anything of the future of the planet, because that would not be *her* future, that would simply be something that happened in the past – possibly in her year 2022: which, lacking any competing observational measurement, would be the only ‘actual’ year of Earth’s destruction.

If she continued to travel straight on without ever returning to Earth, then of course the matter would be plain, since the only calendar she need concern herself with would be that in her spacecraft.

“But she did travel to the future where she found her friend, old but still alive. The point is, she can’t now go back to 2020 and tell Carol.” I think Green could have gone into this more, and perhaps better, because this of course is what people really want from time-travel, fore-knowledge (and control of the past). The mistake here is thinking earth’s time – our time – is absolute. It isn’t; that’s the whole point.

You’re both correct about how the subjects experience different lengths of time, but the issue of date interests me a lot more. If a time traveler leaves earth in 2020, would they increment their calendar daily or would a computer update the computer based on the spaceships speed, acceleration, etc to keep it synchronized with the time on earth? If i were in charge of the bureau of time traveler calendars, I would choose the latter. I have two reasons for this. First, it eliminates ambiguities. If Mary and Carol are talking, they both know that 2022 means two years on earth after Mary left. If Mary wants to know when earth’s 2022 had occured for her, the computer can find that out fairly simply. Secondly, and more importantly in my opinion, calendars work best if there’s one per planet (or star orbitting body). The reason our clocks and calendars are cyclical is because they’re derived from the rotation and orbit of Earth. Imagine you time traveled to Mars, 2300 which has been terraformed to be as full of life as Earth. Would it make sense for martians to use a clock that’s 24 hours long when Mars rotates once roughly every 24 hours and 37 minutes? Would it make sense to use a calendar that is 364 days long when martian years are more than 650 days long? No, a calendar from Earth would only be useful for knowing what date it is on Earth. A calendar is useful for keeping track of seasons that are local to the planet. For spaceships traveling between solar systems, a local calendar isn’t useful. Instead, I would simply have a counter which tracks the number of simulate days since the voyage started. As a tangent to a tangent, i wonder how traditions like birthday and holidays would function on a spaceship not orbitting a star.

“If the earth was destroyed in (Carol’s) 2040, then Mary would still not know anything of the future of the planet, because that would not be *her* future, that would simply be something that happened in the past – possibly in her year 2022: which, lacking any competing observational measurement, would be the only ‘actual’ year of Earth’s destruction.”

I think this is confused. One confusion relates to the way you think about Mary carrying an Earth calendar with her.

If planet Earth was destroyed in 2040 (Earth time: what else does 2040 mean??), it makes little sense to say that it happened “in [Mary’s] year 2022” which you then suggest as the “actual” year of the earth’s destruction because of “the lack of any competing observational measurement.” The point is, Mary was far, far away. You can’t correlate the “nows” of agents that are far apart.

What about all the people who actually (ex hypothesi) experienced the event in 2040? Okay they are no longer around when Mary’s spaceship returns to our corner of the Milky Way galaxy, but so what?

The point is, Mary was not *there* *when* it happened.

She is a witness to the remnants, perhaps, an analysis of which, coupled with an analysis of her own travel might allow her to determine that the earth was destroyed in 2040 (Earth time). But even if she cannot determine this, *if* the earth was destroyed in that year, it was destroyed in that year. I am not talking about absolute time (there is no such thing) but Earth time (as measured by a particular system/calendar).

I also don’t understand this claim: “… Mary would still not know anything of the future of the planet, because that would not be *her* future…”

We are not talking about *her* future, we are talking about the planet’s future (as envisaged by her before her trip).

The planet has its history, its life cycle. Mary left at a particular moment (in what the then-inhabitants called 2020). She returns when it is 2070 Earth time. Maybe it’s business as usual. Maybe there is just a bunch of newly-formed asteroids orbiting where planet Earth used to be.

I must disagree. The traveller will not find herself in a future earth time, since that earth time *is* inaccessible to her. To put it crudely (without the problem of travelling at the speed of light), if I were to travel to the Far East, would I be 4 hours (or whatever) in the future for the people I left behind? Would those people be able to ask me what the future is?

Basically, I made a simple point: that our clocks continue to move at the same rate for us. Why, then, are we constantly told by physicists (in the examples I gave) that “our clocks go slower”? They do not! I am not changing the subject: that IS the subject.

As for Carol and Mary: If Mary had travelled forward in time, it would be fairly natural for Carol to ask her what it is like in the future.

I don’t understand. If she found her friend old and still alive, she did go to back to 2020 Earth time.

I stated that “Carol’s Time had not gone more slowly for Mary. When she came back after five years, those five years were in her past, her memory.” Let me explain: Carol’s time went more slowly *compared to Mary’s time.* For Mary, only 5 years had passed; her memory was of 5 years, not of 50 years.

Yes, she returns to find that the year was 2070 on Earth. But for her, only 5 years had passed, so her calendar shows that it is 2025. Calendars are not universal. Time is not universal; that is the point stated in Einstein’s general law of relativity.

The point of my essay is that we cannot accept that that is what Mary did. Or, rather, we can accept it in science fiction, but it is not backed up by the theory of relativity, despite the fact that it is a natural way of looking at so-called time travel.

Actually, no. She would have learned something about what had happened, not what would still happen. There is no universal time. Time for Mary was running at a rate for her, and at a rate relative to Mary’s time. And if the world ceases to exist, in whose future would Mary now be?

I don’t think that it’s a matter of ordinary usage of language. For x to travel in time, it would mean that x s in y’s future. So if x could ask y, y would be able to tell x what will happen. And we have not even referred to the observer, which I mentioned in my essay.

“For x to travel in time, it would mean that x s in y’s future. So if x could ask y, y would be able to tell x what will happen.”
Here’s your problem. This is not at all what anyone else means when they say time travel.

As my Mancunian relatives would say: “that is effing brilliant, that is”. It’s what I’ve been trying to say on and off for the past twenty-odd years, only with much greater clarity and a lot less getting bogged down in confusing technical detail.

As for Brian Greene, he used to be a physicist, I think. But now he looks to me like carney trash; a peddler of tacky marvels and snake oil. But then, this also looks to be a carney trash age, where people vote for the showmen, not because they believe in it, but because they’ve given up on meaningful change. Kierkegaard once imagined the end of the world as a show, where a clown comes on stage to announce that the theatre is on fire, and everyone thinks it’s a hoot. But I don’t think even he would have imagined that the end of the world was the show, and not because people are laughing, but because they’re at the end of their tether.
Any way back from the brink must of necessity involve eye level communication to reinstate the discourse on facts and their meaning, where values can be expressed as honest opinion, and not as a flourish of a performance.
In that light, simply explaining what time is, and that the marvel is a sleight of hand, or mouth, or mind, is an important task. On its own perhaps not amounting to much, nor reaching as wide an audience as it deserves; but that is the price we must pay for choosing honesty over trash. But if we are to pull ourselves back from the brink, this is the way, one small item at a time. And what is time, then? It is first of all an intrinsic property of all massive objects. You can no more travel through time than you can physically travel outside of your body. But people believe that as well…

Brian Cox is a very different proposition, and in my view a carney man who does not sacrifice honesty and on the level communication; but a science communicator in the vein of Carl Sagan and David Attenborough.
I saw him back in September at the Manchester Arena; ten thousand people, young, old, children, couples on a date; all paying concert ticket prices to watch a physicist talk about the cosmos. The show is a collaboration with comedian and radio talk show host Robin Ince, who provides comic relief. And yet, in the final act, the tables are turned, with Ince delivering a beautiful, moving monologue on the nature of time as a lived, human experience in the relationship between a father and his son; and the handsome rock star physicist is left as a shimmering mirage, of undeniably marvelous, and genuinely marvelous ideas and visions; but also at a remove from the proximate, social and biological reality of being human. And that’s as it should be. Cox of course is in on this. It’s his show.

Yes, and that’s where it belongs. And all the best time travel stories aren’t really about time travel, but about getting the characters to where the plot needs them to be; and that no one ever needed any deeper explanation than timey-wimey stuff.

I agree with you almost entirely, Jesper. But – there always is a but 🙂 – I need to show where Brian Cox, like many other physicists is perpetuating the fallacy that I have been going on about.

In https://www.youtube.com/watch?v=-O8lBIcHre0 he impresses the audience, but he is simply wrong when he states that “Moving clocks move slowly.” No, they don’t. They move more slowly *relative to a clock somewhere else.* “Time passed at a different rate for Jim than it did for the audience watching him move.” No! Time passed at the same rate for Jim, and it would do so whether he moves or not. Time also continued to pass at the same rate for the audience. This is the point I make in my article, that “Clocks moves faster or slower, not for each of us, but relative to an observer. Putting it more verbosely, a clock moving relative to an observer ticks more slowly than an identical clock at rest relative to that observer.” So when Brian Cox finishes with a flourish, saying that “Jim really is a time traveller,” this is, as far as I am concerned, a populist crowd-pleaser, but it is not science.

I apologize to Brian Cox fans. I do not deny that he popularization of science is great, and that the more people are exposed to science, the better. Being popular, though, carries resposibility.

This essay is just one giant straw man. The description of time travel given at the beginning is not the one used by any of the presenters in the video sources provided.

“We can’t slow down or speed up the rate of our personal time. Our clocks and watches do not go slower or faster for us, no matter where we are or how fast we travel.”
This makes it sound like you have never seen a time travel movie. Every book i’ve read and film i’ve seen with time travel in it does not modify how quick someone’s personal time flows (because that is nonsensical). In Back to the Future, the time traveling DeLorean disappears and then reappears in another time. Marty didn’t experience his personal time speed up, slow down, or flow in reverse. The shot from inside the car shows this plainly (https://www.youtube.com/watch?v=KPeHFDxKUP4). At the end of BttF Marty travels 30 years forward in a moment which isn’t physically accurate, but it’s easy to imagine a similar situation which would be. Instead, Marty could use the spaceship Mary used in your example. The only difference is the rate of time travel, near instantaneous for the DeLorean vs gradual for the spaceship.

“That would be quite a shocking reunion, but somewhat mitigated by Carol thinking she could ask Mary – who, theoretically, had time-traveled – what was going to happen in the future, perhaps to know the winning lottery numbers that were still to come. Mary would look at her in amazement, for lottery numbers that had come up were obviously not in Carol’s future.”
This example is also completely at odds with time travel movies. In BttF2 Biff travels back in time to give a book with records of future game results which he uses to bet and make money off. Now imagine Biff had traveled forward in time. The same book would be worthless to future Biff because those games already happened. You implied that time travelers know what’s going to happen in the future, but that obviously only true for time travelers who traveled to the past.

“Brian Cox says that “moving clocks move slower.” (3)They don’t. They do so only in comparison with other clocks moving at different speeds.”
This is like repudiating a child who says giraffes are tall by saying that they’re only tall relative to other animals. Trying to evaluate slow or tall without something to compare it to makes no sense.

“This fact, he says, has nothing to do with Einstein’s theories. (6) But it is not fact, and it has everything to do with Einstein’s theories.”
This isn’t even what he said. Starting at 20:46 (https://youtu.be/N-NTXoYTvao?t=1246) he says, “This is not a funny consequence of a string theory dreamed by Einstein.” He’s making it clear that the effect he’s describing is real and measurable, not just theoretical. Next you claim the effect of keeping atomic clocks at different altitudes isn’t a fact. Unless you have are going to claim that Chou, Hume, Rosenband, and Wineland (https://science.sciencemag.org/content/329/5999/1630) fudged their data, I can only assume you just don’t know what you’re talking about.

Thank you for your comments, Soren. I am somewhat bemused that you are using science fiction as a means of refuting my arguments. It was exactly my point that time travel works well in science fiction, which is where it belongs. The wonderful thing about fiction is that you can make anything happen, so it is no wonder that time travel is so popular in that genre. I have read and watched a lot of SF and enjoy it in its context. But it is not based on science.

You say that I “implied that time travelers know what’s going to happen in the future, but that obviously only true for time travelers who traveled to the past.” I didn’t. I didn’t because time travel is not feasible within the theories of physics. Time travelers cannot know what is going to happen in the future any more than any of us can. And how would anyone know that they had (had!) if they don’t go back to where they had come from? How would Biff in that SF movie know, if he didn’t return to where he started out from? And – if you wish to use SF as an example – when he got back, the future was in his past, not in his future, nor in his parents’ future.

Time can only be relative to other time being measured elsewhere. Time is special, as worked out by Einstein. There is no analogy to giraffes or anything else. Time is special because it is not universal.

As for what Carlo Rovelli actually said, at 21.18 in the video is as I quoted”…so it’s a fact. It has nothing to do with Einstein’s ideas…” (sorry, I wrote” theories” instead of “ideas”.) Of course Rovelli is describing what is measurable. But he is making a fundamental mistake by saying that clocks move at a different rate depending on their height.

So when you state that “Next you claim the effect of keeping atomic clocks at different altitudes isn’t a fact.” Not so. This is what I wrote:

“While time does go faster on Earth at a higher elevation, relative to an observer, it does so due to gravitational time dilation, a consequence of Einstein’s general theory of relativity, which posits that the gravity of a massive body – such as the Earth – warps the space-time around it, causing the flow of time to speed up or slow down depending on its distance from the mass.”

My point – and this is the theme of my essay – is that each clock itself does not move faster or slower. They do so relative to each other, in line with Einstein’s theory of relativity. It doesn’t matter how accurate the clocks are, they run relative to each other. So yes, it is measurable that a clock on a mountain goes faster than a clock in a valley (or only 30 cm difference), but the clocks themselves do not go faster or more slowly.; And so it has everything to do with Einstein’s ideas. It’s not a matter of scientists fudging their data, but of us understanding what the relativity of time means. And physicists who explain their subject to the public should do so accurately even if it means that it is less populistic.

Strawman? Okay, that’s not what I thought. This is not about the science, nor whether Cox, Greene et. al. understand the science: of course they do. Nor is it about time travel as a literary device. Whether your plot needs a Tardis or a wardrobe, that’s all the same in terms of substance; these are plot devices, and nothing matters less in that context than getting the physics right. As they say on TV Tropes, time travel is not allowed to make sense.
This is about science communication, and whether then ‘time travel’ or ‘clocks slowing down’ constitute fair use of those common language phrases. Ronald Green thinks not, and I agree with him. Your clock is not slowing down, and living your life does not constitute time travel.
The trouble with this is that people then get it wrong, whether greatly inflating the importance of relativistic effects in everyday life, or turning away from science (or turning into a lifetime of claiming that “Einstein was wrong”) because this doesn’t make any damn sense.
One science communicator who does get right is Don Lincoln of the Fermi lab YouTube channel.

This seems to miss the point. General relativity allows closed time-like curves. Any recent time machines described by physicists like Kip Thorne, Robert Gott or Robert Forward, as well as those appearing in recent SF, involve traversable wormholes. If you search on recent citations of Hawking’s Chronology Projection Conjecture, you will find 96 papers since 2018.

Quite, but then it is becoming increasingly clear that spacetime simply isn’t fundamental, as it has no operational meaning at high enough energies.
Also, as we all know, all inquiries are framed, and starting in the sixties with Stephen Weinberg and others, it is equally becoming clear that all frames in physics has a scalar component. Humans are much bigger than elementary particles, and much smaller than galaxies or supermassive black holes, so no one has any a priori right to assume that the rules are the same, and in general we will find that not even the things we can talk about are the same. It makes no sense, for example, to predicate consciousness of electron, because consciousness has no operational meaning at the scale of the electron. I may not know what consciousness is in any way that I can give as a succinct definition, but at the human scale I can point to you and say, “There. That’s what I’m talking about!”, so operationally I’m on secure footing.
Likewise, free will, whatever it may be, has no operational meaning at the scale of supermassive black holes. So to explain any of these things in terms of a traveler is, quite simply, an abus d’language.
Einsteins equations do contain closed timelike geodesics as solutions, but the actual universe does not appear to contain any such, and in any case Einsteins equations are an approximation to a deeper theory that shall have spacetime emerge from statistical mechanics and consistency requirements on elementary particle interactions.
The physicist who talks most clearly on these matters is in my view Nima Arkani-Hamed.

The first is a public lecture, so should be accessible to everyone. The second is the introductory lecture at a Harvard course. It is technical and somewhat challenging.

Glad to be thinking about this! However, I disagree strenuously with your argument.

If Mary says she wishes she could “time travel into the future,” we can ask what that means to her. Here, I think, are some plausible reasons she might give: a) she wants to know what her home town will look like 100 years hence, b) she wants to live in a time where several enemies of hers have died of old age, c) she wants to be there when the time capsule her town buried will be dug up, d) she wants to leave money in an interest-bearing account and come back when it has grown, or e) she’d like to tell people in the future what it was like to live through the 2020 election. Unfortunately, she will not live to see or do any of these things. I take it these are the kinds of reasons people speaking naturally might give for wanting to travel (permanently) into the future. All of these could be achieved by the near-light-speed travel you imagined in your article! I find it very unlikely that Mary would give the reason *f) she wants to experience my personal clock moving at a different speed.

I don’t think you’re intentionally attacking a straw man as some have suggested, but I agree with those posts above saying that you are attacking a position no one holds, or at least not what the physicists you mention, science fiction writers, or the general public intend when they talk about “time travel.” Do you truly think that most movies or books that posit time travel think about it in the way you assume? Can you give examples?

Thank you for your comments, ianwerkheiser. I don’t, though, understand what strawman I am supposedly attacking. Surely the notion of time travel isn’t a strawman. I am attacking the very notion of time travel and those physicists who (1) believe/claim that time travel is possible even in theory, and (2) don’t seem to understand what time travel is and the repercussions of the notion itself.

I don’t care what the motivation or reasons of so-called time travelers is. The fact that we would all like to know what the future holds is,as I mentioned in the article, the reason that time travel is such a popular scenario.

By the way, I don’t understand your d and e reasons. Reason d seems to imply that the person will be revived from the dead in order to see what had happened to her money. Surely, she will see what has been happening just by living her life. In scenario e, surely people will know from history what happened in the 2020 election. Perhaps you mean that someone would want to travel in time in order to see what will happen in the 2025 election. But that’s the point – by traveling at close to the speed of light for 5 years on your clock, she will not know what has happened in the future of those she left behind on earth. Her 5 years will not encompass 5 years within the 50 years that elapsed on Earth.

I also don’t understand your point about Mary wanting “to experience [her] personal clock moving at a different speed.” Of course she wouldn’t see that. That was the specific and central point I was making in my article: that one’s personal clock continues as normal. It does NOT move faster or slower; it does so only relative to clocks elsewhere in the universe, which can be seen only by an observer.

The notion of time travel is difficult one. SF books and films make it easy and fun. Just to give an example of what traveling forward in time would mean: Someone x who travels forward in time would be there when someone y eventually arrives there. Now let’s say that x travels back to where he started from and then travels forward again to where he had traveled to. In that case, he would find himself there (because it’s the future). So there would now be two of himself! I haven’t even mentioned the fact that by doing it a second time, the future would be also his past.

For physicists to talk blithely about time travel without understanding what time travel actually means, is either a case of fooling themselves or taking the role of pop stars in a populistic world of bells and whistles. Furthermore, for physicists to lay out mathematical formulae to show the reality of time travel, is simply to show that mathematics works. It shows the reality of mathematics, but not of time travel.

We might be talking past each other. First let me clear up a few points and then I’ll make another run at it:

You say you don’t understand D and E, and indeed your guesses show that there was a miscommunication. By D, I meant that she has placed money in an interest-bearing account. Over time, this account will accrue a lot of money which is quite nice for her (assuming it increases faster than inflation). However, she is impatient and doesn’t want to wait. What she would like to do is travel to a point in the future where there is a lot of money in that account, but get there without having to experience all the waiting that sitting in the bank would require. By E, I meant that she would like to provide future historians with a first-person account of the 2020 election. Given that current historians would love to have first-person accounts of moments in the past, it is reasonable for her to expect that historians in the future would love to have that as well. Unfortunately, she will die of old age before those memories are valuable. She wishes for a way to go to a point in the future where her memories and experiences would be valuable to others, but without having to wait and age at a normal pace.

You mention these as a “by the way” sort of question, but I actually think they strike at the heart of the matter: namely, that when people refer to “time travel” into the future, this is the kind of thing they mean by this term. Given that these could be accomplished by near-light-speed travel in a space ship (or at any rate assuming that they could be), then physicists are right to say that time travel into the future without going back again (as people usually conceive of the idea) quite possibly comports with the laws of physics. When you say that time travel is not possible, you are referring to a different sense of time travel than are those physicists, or indeed are most people. So a better gloss on your argument might have been “there’s a particular kind of time travel that is impossible.” But in that case, there are quite a lot of particular kinds of time travel that is impossible, so you’ve merely described another.

I think this objection is at the root of quite a number of the other comments to this post, which actually is (weak) evidence that I am right when I say that the kind of time travel you are attacking is not the kind people standardly refer to when they use that phrase.

First of all, ianwerkheiser, let me say that I appreciate this exchange. I am always amazed by exchanges about (what should be) non-personal issues becoming acrimonious.

I have been giving a lot of thought about a constant theme here about ‘different kinds of time travel’ and that other people (‘most other people’) refer to time travel differently to the way I do. I find that surprising, since I don’t know of any other way that time travel can be referred to. Please don’t think that I am being arrogant or presumptuous by saying that. So let me explain.

If we forget for the moment why I believe that the very notion of time travel is fallacious (as per my article), I would like to set out what time travel actually means and the repercussions therefrom. It can only mean one thing: traveling to the future (forgetting about traveling to the past, which has different problems). There is no other meaning. If I may give an analogy regarding ‘nothing’, which I have also researched extensively. Nothing (the absence of everything) means exactly that. It does not mean what some physicists (like Lawrence Krauss) call ‘nothing’ so that they can explain how the universe was formed from that ‘nothing’. But that ‘nothing’ is something. If I were to write an article in this forum about ‘nothing’, I would be attacked in a similar way as I am about time travel. I would be accused of referring to nothing in a different way from which scientists do.

Having the cake and eating it too is what this is about. Nothing, the absence of everything (including ourselves) has no potential, including the potential for creating a universe. I would be attacked for that statement, since physicists would show the mathematics and details about QM in order to demonstrate that a universe not only can, but actually did, come from nothing.

So with time travel. I am aware of the theories of curved spacetime, causal set theory, loop quantum gravity, and string theory, etc., all showing the theoretical possibility of time travel. In all of this, though, what is missing is an understanding of what time travel actually means. Mathematics is not time travel, even if the equations are beautiful. Showing that a particle can (does?) ‘go back or forward in time’ does not show an understanding of what that means. Going back in time (as ridiculous as the very concept is) means that the particle (or whatever) goes back to where it had been before, so meeting itself.

But let’s leave backward time travel. You – quite rightly – wish to discuss traveling to the future. In which way is my understanding of what that means different to what people “standardly refer to”? Taking your example of what going forward in time to see what will happen to your bank account would mean, let me set out some repercussions of traveling to the future:

– Traveling to the future means that you are going to a time that will happen. Not *might* happen, or *probably* happen, but WILL happen. It means that if you travel to the future, you will meet yourself because you have traveled to your own future.

– Whose future will you get to? You say that you will know what will happen in the place you left behind. But when you get to the future, it will be to something that will have happened for you. How will it be the future of the person (you) who is still there? Are there now two of you? Well, no, because there is no you left behind.

– How will you know that you have traveled to someone’s future? Only by comparing the two times. Your caveat here is to stipulate that the person doesn’t return. So who knows about the future? The answer is the stipulation in the relativity of time that it is relative to an observer. If there is no observer, there is no way of showing the 2 times. And where would the observer be?

The above scenarios are pretty bizarre. Crazy, even. Of course they are, since the whole notion of time travel is crazy.

You make the by-the-way remark that: “Given that these could be accomplished by near-light-speed travel in a space ship…” No, it is not a given. The point of my article is that traveling at close to the speed of light is NOT traveling in time, and I showed why not, in that it does not correspond to the laws in physics that make the notion itself feasible. The discussion we are having at the moment is what time travel means.

It does seem, unfortunately, that “When you say that time travel is not possible, you are referring to a different sense of time travel than are those physicists…” That is certainly problematic, since physicists are concerned with mathematics, and do not stop to think that time travel means traveling to the future and that that has repercussions.

My argument is not that “there’s a particular kind of time travel that is impossible.” Any and all travel to a future (that is still to come) is impossible, even conceptually when the basis of what that could mean are understood.

I am wondering what physicists mean when they use the phrase time travel that does not take into account the meaning understood by SF and non-physicists.

I have a great respect for physicists, but often they seem to have painted itself into a corner. Isolating themselves into the be-all and end-all of sometimes-untestable theories (such as the many-worlds theory, that allows different worlds to account for and justify the grandfather paradox no longer being a paradox but an alternative reality.)

As with ‘nothing’, so with ‘time travel’, it would be a boon for physicists to get together with philosophers in order to look at the world within a wider perspective. Perhaps philosophers can attempt answers to issues that scientists don’t think about. And vice versa, of course.

I don’t suppose my views will be too popular with physicists. I don’t really understand why, but from the discussion on this forum, I get a glimpse of the problems that are called ‘speaking past each other.” It would be nice if we could speak to each other instead.

Again, I want to thank you for opening the discussion. The whole discussion and comments have been somewhat of an eye-opener for me (although it isn’t the first time I have had to battle with scientists). And I thank The Electric Agora for giving us the opportunity to see other points of view.

Consider Jack walking from the last carriage of a train, where he boarded at one station, to the front carriage, where he then alights at the next station. It seems to me that he did not “slow down or speed up” his experience of walking, but he does arrive in a novel environment distant from his origin, and does not see his friends, who decided to walk the entire distance. You will have to explain to me where the (Galilean v. Einsteinian) analogy breaks down, which I think drives the usual understanding of “forward” time travel, whether by Uzair, Sleeping Beauty, Rip Van Winkle etc.

However, I think “real” time travel always involves backward travel, and that the only way it can be rationalized is eternalism – a point of view that was commonly drawn as the implication of Minkovski’s formulation. One can always see backward time travel as de novo creation of matter and information – we don’t have to actually believe the backstory the putative time traveller gives us. Maybe he’s just a Boltzmann Brain.

Thank you, Davidlduffy, for the question and comments. I don’t, though, understand why you are asking me about the train scenario. I have never said that Einstein was wrong, nor have I argued with some of the physics that has been mentioned. My point – my only point – was that those theories have nothing to do with time travel. The fact that there is no universal time and that time is relative according to location, is not traveling in time.

As for “the usual understanding of “forward” time travel”, it is a fantasy, and a very entertaining one, but not based of theories in physics. The title of my article is “The Fallacy of Time Travel.”

In your train example, why would you think I have a beef with Galileo or Einstein? Are you saying that Jack or his friends time traveled? Is Jack in his friends’ future at any point? Could they ask him what is going to happen in their future?

As for backward time travel, I was under the impression that that notion was discarded long ago. Didn’t the grandfather paradox make that clear? How is de novo creation of matter and information backward time travel? Traveling back in time surely means that one goes to what already was.

The concept of time travel is clear. Traveling back in time means going to events that have already happened, while traveling forward in time means going to events that have not yet happened.

Hold your horses — I thought Zephram Cochran is due to invent the warp engine for faster-then-light travel in the 2050s. Besides, don’t the Vulcans already possess it?? It has always amazed me how rigidly adherent to reason, physical law, and a strictly material worldview physics popularizers and science nerds in general behave — and also stridently advocate for — except when it comes to their favorite beloved romantic sci-fi notions they became enraptured by during adolescence and still have no clue about letting go of. “We will find the time travel loophole because I want it to exist and I have a real bad itch to see the technological utopian future when all boring social matter have been resolved or eclipsed and people have STEM classes in embryo.” Bravo to you for thinking through the relativity paradox on a down-to-earth basis!

You’re right, davidlduffy. And I think you have hit the nail on the head in relation (sic) to our discussion. I have no problem with moving spacially. Nobody would. Changing locations being relative to other locations is not a conceptual problem (at least not at the macro level). But time is another kettle of fish; people are still arguing about its very definition, or if it even exists. So much more so for the notion of time travel, that requires ‘movement’ to a period that has already taken place, or to a period that hasn’t yet happened. Those are mind-numbing concepts, that have been simplified, i.e. trivialized, taking good theories of physics that work and then seeing in them consequences that do not apply.

Sticking within the parameters of the popular interpretation, one can even conceptually engineer the thought experiment to enable travelling to the past, by reversing Mary and Carol. Imagine that Carol, and the entire planet, accelerate away from Mary, at near the speed of light, and then return, five years later (in Carol’s time. For Mary it will have taken 50 years for the Earth to return). Mary will have aged to 75, but Carol, and the rest of the planet, will only have aged 5 years. For Mary it is now 2070, but for Carol and the rest of the Earth, it is only 2025 (let’s ignore all the logistical problems involved in the Earth moving away at near the speed of light, for the sake of the discussion). Mary could easliy convince herself that she had travelled into the past – her past – as Carol, her friends, the newspaper dates, and indeed the entire globe were still in what for them was the year 2025, while she (Mary) had, according to her calendar, passed this date long ago. Mary, once the Earth has returned, appears to have suddenly gone from 2070 back to 2025! So it would appear that the very same thought experiment that supposedly allows travel to the future, also allows a limited form of travel to the past.

It seems to me that the thought experiment can be manipulated *either* way, and so doesn’t do what the popular presentations claim, that is show that there is somehow a kind of factual loophole that demonstrates that while travel to the past may be impossible, a sort of time travel to the future can’t be ruled out. If one accepts the first situation (Mary travelling to the ‘future’), one has to accept the other (Mary travelling to the ‘past’). The explanation is that both of them only work because, socio-culturally, Earth time is always absolute. Whatever year it is on Earth is what year it ‘really’ is. Of course, as Ronald Green has been tirelessly pointing out, no real time travel has occured at all, in either situation. It is only because time is at least in part determined by social and cultural references, and is so in this way only, absolute and not relative, that these thought experiments work at all. The time on Earth will always be a priviliged frame of reference, even though, stricly speaking, there is no absolute frame of reference. I think it might be easier to see this if we keep in mind the the reversed thought experiment above.

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